Semiconductor memory devices are generally classified into volatile and nonvolatile types. Volatile semiconductor memory devices are operable at high rates of reading and writing, but can lose their data when there is no power supply. But, nonvolatile semiconductor memory devices are able to retain their data even without power supply. Thus, nonvolatile semiconductor devices are widely used in various applications that require data retention regardless of power supply. There are many kinds of nonvolatile semiconductor memories, e.g., mask read-only memories (MROMs), programmable ROMs (PROMs), erasable and programmable ROMs (EPROMs), electrically erasable and programmable ROMs (EEPROMs), and so forth.
Generally, it is inconvenient for users to update information from MROMs, PROMs, and EPROMs because those memories are not easily erased and written with new data. In contrast, EEPROMs are able to be electrically erased and written with data, and are increasingly used as subsidiary storage units or system programming tools that necessitate steady data updates. Moreover, flash EEPROMs can be fabricated with higher integration density than traditional EEPROMs, which make them more useful in applications for large capacity subsidiary memory units. NAND-type flash EEPROMs (hereinafter, referred to as ‘NAND flash memories’) can be more advantageous to integration density than other types of the flash EEPROMs (i.e., flash memories).
In flash memories, a storable (permissible or available) programmed state of a memory cell is determined by the number of bits stored in the memory cell. A memory cell storing 1-bit data (or a single data bit) is referred to as ‘single-bit cell’ or ‘single-level cell’ (SLC). A memory cell storing multi-bit data (or two or more data bits) is referred to as ‘multi-bit cell’, ‘multi-state cell’, or ‘multi-level cell’ (MLC). With increasing demands for high-density memory devices in recent years, many studies are proceeding about multi-level flash memories capable of storing multi-bit data per memory cell.